The primary aim of this study is to develop a registry for Hereditary Hemorrhagic Telangiectasia (HHT) to characterize systematically the clinical presentation, the prevalence of particular manifestations and the natural history of this disorder, linking these phenotypic subtypes to particular genetic defects that have been and are in the process of being identified. The secondary aim is to determine the prevalence of pAVM (pulmonary arteriovenous malformations) in asymptomatic patients with HHT, to study if certain patient subgroups or families are at increased risk of pAVM and to document the sensitivity and specificity of commonly used screening tests for the detection of pAVM.